<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        //用户输入数字n
        var n = Number(prompt('请输入数字'));

        //累加器
        var sum = 0;
        //遍历分母
        for (var i = 2; i <= n; i++) {
            sum += (i + 1) / i
        }

        alert(sum.toFixed(2));
    </script>
</body>

</html>